Ag Nanohole Arrays Research Articles

Enhanced trion emission from WS2 monolayers directly exfoliated on Ag nanohole arrays

The extraordinarily large exciton binding energies of transition metal dichalcogenides (TMDs) have attracted significant attention in both fundamental scientific research and innovative technology development. TMD-metal nanostructures have been fabricated to control excitonic behaviors as well as improve the light-matter interaction in TMDs. WS2 monolayers are integrated with periodic Ag nanohole (AgNH) arrays prepared using a template stripping method. The ultrasmooth and contamination-free surface of the template-stripped Ag layer allows for the direct exfoliation of WS2 flakes on the AgNHs. Surface plasmon excitation increases optical absorption in WS2/AgNH, as evidenced by reflectance, PL, and Raman measurements, along with numerical calculations. Furthermore, the AgNH structures significantly increase the trion-to-exciton emission ratios from the WS2 monolayers on them. Nanoscopic surface photovoltage mapping of WS2/AgNH using Kelvin probe force microscopy can visualize electron accumulation at the suspended WS2 region under light illumination, which triggers the formation of trions. All the results highlight the capability of WS2/AgNH to boost trion emission through plasmon-induced concentrated light and a built-in potential gradient. This work introduces a simple and efficient strategy for fabricating TMD-metal integrated systems to control their excitonic behaviors.

Inverse Design of Plasmonic Nanohole Arrays by Combing Spectra and Structural Color in Deep Learning

Herein, deep learning (DL) is used to predict the structural parameters of Ag nanohole arrays (NAs) for spectrum‐driving and color‐driving plasmonic applications. A dataset of transmission spectra and structural parameters of NAs is generated using finite‐difference time‐domain (FDTD) calculations and is converted to vivid structural colors using the corresponding transmission spectrum. A bidirectional neural network is used to train the transmission spectrum and structural color together. The accuracy of predicting the structural parameters using a desired spectrum is tested and found to be up to 0.99, with a determination coefficient of reproducing the desired spectrum and color to be 0.97 and 0.96, respectively. These values are higher compared to those when only training for spectrum, but requiring less training time. This strategy is able to inverse design the NAs in less than 1 s to maximize surface‐enhanced Raman scattering (SERS) enhancement by matching transmission resonance and laser excitation wavelength, and accurately regenerate colored images in 7.5 s, allowing for nanoscale printing at a resolution of approximately 100 000 dots in−1. This work has important implications for the efficient design of nanostructures for various plasmonic applications, such as plasmonic sensors, optical filters, metal‐enhanced fluorescence, SERS, and super‐resolution displays.

Bloch-Surface Plasmon Polariton Enhanced Amplified and Directional Spontaneous Emission from Plasmonic Hexagonal Nanohole Array.

The light-matter interactions at nanoscale can be enhanced by Bloch-surface plasmon polariton (Bloch-SPP) on the plasmonic lattice. An Ag nanohole array in hexagonal arrangement served as an optical cavity to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. A 100-fold enhanced ASE was observed at 15° emission angle under TM polarization when the pump power density exceeded the threshold of 198 W/cm2 based on the degenerated high state density modes. Moreover, a specific polarization dependence of ASE was modulated by the Bloch-SPP modes, and the degree of polarization was enhanced from 1.3 to 2.1 when the pump power density exceeded the threshold of ASE. This work clarifies the interaction between the gain media and plasmonic systems, which lays a foundation for the plasmonic device designing.

Directional amplified spontaneous emissions from Ag nanohole array with high diffraction orders.

Surface plasmon excitations in metallic hole arrays have been extensively studied in the context of light-matter interaction, since the generated Bloch surface plasmon polariton (Bloch-SPP) exhibits unique properties of nanoscale light confinement, near-field enhancements, and long-range metal surface propagation. In this work, we experimentally demonstrate a plasmonic device that exhibits highly directional emission in visible light; using Ag film with a thickness of 100 nm deposited on a subwavelength porous alumina array as a plasmonic cavity, four-level rhodamine 6G (R6G) is selected as the gain material. It is suggested that a Bloch-SPP with high diffraction orders on a Ag nanohole array can generate a strong local electric field and a high Purcell factor at a nanohole edge. Moreover, directional five-fold enhanced amplified spontaneous emission (ASE) with polarization dependence is observed under a low threshold of 199.9 W/cm2 in the visible light region, which comes from the optical feedback provided by the 2D periodic nanohole array. This work opens up a wide range of applications for real-time tunable wavelength, controlled multimode laser, fluorescence detection, etc.

Altering the distribution of excited-state lifetimes in aminated GFP chromophores by Ag nanohole arrays

Fluorescence of the modified GFP chromophore diethyl-ABDI-BF2 dispersed in PMMA matrix is studied on top of glass, continuous and perforated optically thin silver films. In polymer, the fluorescence decay kinetics becomes non-exponential and can be described by the distribution of rate constants. The results demonstrate shortening of the excited state lifetime in the presence of silver and broadening of the lifetime distribution caused by the nanoholes.

Extraordinary sensitivity enhancement of Ag-Au alloy nanohole arrays for label-free detection of Escherichia Coli.

Alloy nanostructures unveil extraordinary plasmonic phenomena that supersede the mono-metallic counterparts. Here we report silver-gold (Ag-Au) alloy nanohole arrays (α-NHA) for ultra-sensitive plasmonic label-free detection of Escherichia Coli (E. coli). Large-area α-NHA were fabricated by using nanoimprint lithography and concurrent thermal evaporation of Ag and Au. The completely miscible Ag-Au alloy exhibits an entirely different dielectric function in the near infra-red wavelength range compared to mono-metallic Ag or Au. The α-NHA demonstrate substantially enhanced refractive index sensitivity of 387 nm/RIU, surpassing those of Ag or Au mono-metallic nanohole arrays by approximately 40%. Moreover, the α-NHA provide highly durable material stability to corrosion and oxidation during over one-month observation. The ultra-sensitive α-NHA allow the label-free detection of E. coli in various concentration levels ranging from 103 to 108 cfu/ml with a calculated limit of detection of 59 cfu/ml. This novel alloy plasmonic material provides a new outlook for widely applicable biosensing and bio-medical applications.

Large-Area Fabrication of Complex Nanohole Arrays with Highly Tunable Plasmonic Properties.

By combining nanosphere lithography with oblique angle deposition, large-area asymmetric compound Ag nanohole arrays with nanorods inside the hole were patterned on substrates. The technique enabled the production of complex nanohole arrays with controlled hole diameter, thickness, and rod structure inside the hole. The compound asymmetric Ag nanohole structures showed strong polarization-dependent optical properties, and a new extraordinary optical transmission (EOT) mode with tunable resonance wavelength at the near-IR region was observed. The transmission at the new EOT wavelength region can increase from 27% of nanohole to 69% of the compound structure, and these structures can achieve a refractive index sensitivity as high as 847 nm RIU-1. The tunable EOT wavelength and strong polarization-dependent optical properties make the structure ideal for ultrathin optical filters, polarizers, surface-enhanced spectroscopies, etc.

Enhanced photoluminescence of DCJTB with ordered Ag-SiO2 core–shell nanostructures via nanosphere lithography

We have fabricated Ag-SiO2 core–shell nanostructure set in the hexagonally ordered Ag nanohole array by nanosphere lithography with reactive ion etching, and followed by Ag deposition. The resulting nanostructure includes the triangular-shaped plates with sharp edges on the top, the Ag-SiO2 core–shell nanospheres, the hexagonally arranged nanohole array, a SiO2 buffer layer and a DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) fluorescent dye layer, respectively. Six patterns of substrates (i.e., Types 1–6) were fabricated and their photoluminescence enhancement performance was compared to substrate layered with pure DCJTB. The experimental results show that photoluminescence enhancement can be 15.69 times, and the lifetime can be shortened from 0.97 ns to 0.41 ns for Type 3 when compared to the pure DCJTB one. The finite element method revealed four structure conditions which are the effects of edge enhancement, gap plasmon resonance, hollow plasmon resonance, and core–shell hybridization plasmon resonance and can contribute to the photoluminescence enhancement factor. The proposed substrates provide a practical detecting platform with plasmon-enhanced photoluminescence, and the fabrication methods used are technically simple and low cost.

Hybrid octahedral Au nanocrystals and Ag nanohole arrays as substrates for highly sensitive and reproducible surface-enhanced Raman scattering

A 3D SERS substrate with a hybrid structure of octahedral AuNCs and AgNHs exhibits high enhancement and reproducibility.

Highly reproducible and stable surface-enhanced Raman scattering substrates of graphene-Ag nanohole arrays fabricated by sub-diffraction plasmonic lithography

Surface-enhanced Raman scattering (SERS) has shown great promise for trace detection due to its high sensitivity. The lack of efficient fabrication methods for large-scale SERS substrates with high sensitivity, high reproducibility, and stability has greatly limited the development of practical SERS sensing devices. In this work, we report sub-diffraction, high throughput, and low-cost fabrication of SERS substrates with plasmonic cavity lens (PCL) lithography. The PCL is a photoresist layer sandwiched with two Ag layers, which could greatly improve the lithographic resolution and fidelity. Ag nanohole arrays (AgNHs) over a 5 × 5 mm2 area were fabricated onto the bottom Ag reflective layer, which worked as SERS substrates. The SERS substrates exhibited enhancement factors (EFs) of 107 that are capable of monolayer detection. The reproducibility is less than 9%, which is much better than that of substrates synthesized with traditional chemical methods. The graphene (GE) layer was transferred onto AgNHs to increase the SERS stability, as demonstrated with only a 25.8% decrease of SERS intensity for 90 day exposure to air and a 78.3% decrease for the control case without GE. This work has demonstrated that PCL lithography technique is a promising method for fabrication of SERS substrates with large-area, high sensitivity, high reproducibility, and low-cost.

Dipole Radiation-Induced Extraordinary Optical Transmission for Silver Nanorod-Covered Silver Nanohole Arrays

A new extraordinary optical transmission (EOT) mode is discovered when a hexagonal Ag nanohole array is covered by a tilted Ag nanorod array. The resonant wavelength of this EOT mode red-shifts with respect to the normal EOT mode predicted by plasmon-grating coupling theory or dynamic scattering theory and increases linearly with the nanorod length. The structures also show strong polarization dependence; i.e., when the electric field (E-field) direction of the incident light is perpendicular to the long axis of the nanorods, only normal EOT spectrum is visible, but when the E-field is parallel to nanorod axis, the new mode appears. Our finite-difference-time-domain calculations confirm this finding and show that this new mode is due to the dipole radiation of the nanorods on top of the nanoholes. It is expected that other complicated unit cells of nanohole arrays could generate other new EOT modes and can be used to design new optical devices.

Electrolytic deposition of TiC nanoparticles/Ag composite films and their optical properties

TiC nanoparticles/Ag composite films were successfully prepared on an indium tin oxide (ITO) substrate through co-electrodeposition, using the Ag plating solutions with minor addition of TiC nanoparticles, followed by heat treatment in vacuum. The X-ray diffractometer, scanning electron microscope and UV–Vis spectrophotometer were used to characterise phase composition, morphologies and optical properties of as-fabricated films, respectively. Experimental results show that only TiC and Ag phases are identified for the TiC nanoparticles/Ag composite films and TiC nanoparticles are incorporated tightly and uniformly on the surface of the composite films, without obvious clustering. The results of optical property test reveal that the TiC nanoparticles/Ag composite films have similar optical properties as thick silver films perforated with nanohole arrays, which will pave the way towards numerous potential applications of Ag nanohole arrays.

Quantitatively extracting the contribution of asymmetric local-field to χ(2) in cross-shaped Ag nanoholes.

We systematically study the contribution of local-field distribution to second-harmonic generation (SHG) in cross-shaped Ag nanohole arrays, which is usually covered by resonance enhancement effect. By increasing one arm-length of the centrosymmetric cross-shaped Ag nanohole, the local-field distribution varies from centrosymmetric to non-centrosymmetric, while the localized surface plasmon resonance peak is red-shifted to the wavelength of the pumping laser accordingly. Both experimental and stimulated results indicate that the contribution of the asymmetric local-field distribution to SHG is quantitatively separated from a strong resonance enhancement effect. It shows that the pure effective second-order nonlinear susceptibility increases as the asymmetric degree of local-field distribution increases, and the largest effective second-order nonlinear susceptibility is ~2.5 times to that in a centrosymmetric local-field distribution. Our results provide evidence for optimizing the design of nonlinear plasmonic nanoantennas and metasurfaces.

Broadband optical magnetism in chiral metallic nanohole arrays by shadowing vapor deposition

We show that broadband optical magnetism can be achieved through incorporating multi-scaled 3D metallic meta-elements into Z-shaped nanohole arrays. The broadband effect arises from the excitation of multiple magnetic resonances in the meta-elements at different wavelengths. Moreover, the nanohole arrays exhibit a large transmission difference for left- and right-handed circularly polarized incident light due to the chiral arrangement of the meta-elements. More importantly, we have realized experimentally the broadband behavior for the optical range in Ag nanohole arrays fabricated by using a shadowing vapor deposition method. Our study opens up new opportunities for achieving broadband artificial magnetism at visible frequencies which allows possible applications in plasmonic bio-sensors or energy concentrators.

Broadband light absorption of silicon nanowires embedded in Ag nano-hole arrays

Silicon nanowires (SiNWs) embedded in Ag nano-hole arrays with broadband light absorption is proposed in this paper. Finite Difference Time Domain (FDTD) simulations were utilized to obtain absorptivity and band diagrams for both SiNWs and SiNWs embedded in Ag nano-hole arrays. A direct relationship between waveguide modes and extraordinary absorptivity is established qualitatively, which helps to optimal design the structure parameters to achieve broadband absorptivity. After introducing Ag nano-hole arrays at the rear side of SiNWs, the band modes are extended into leaky regions and light energy can be fully absorbed, resulting in high absorptivity at long wavelength. Severe reflection is also suppressed by light trapping capability of SiNWs at short wavelength. Over 70% average absorptivity from 400 nm to 1100 nm is realized finally. This kinds of design give promising route for high efficiency solar cells and optical absorbers.

Interplay between absorption and radiative decay rates of surface plasmon polaritons for field enhancement in periodic arrays

We studied the effects of absorption and radiative decay rates of surface plasmon polaritons on the field enhancement in periodic metallic arrays by temporal coupled mode theory and finite-difference time-domain simulation. When two rates are equal, the field enhancement is the strongest and the peak height of the orthogonal reflectivity reaches 0.25. To demonstrate this fact, we fabricated two series of two-dimensional Au and Ag nanohole arrays with different geometries and measured their corresponding reflectivity and decay rates. The experimental results agree well with the analytical and numerical results.

Plasmonic Metals for Nanohole-Array Surface Plasmon Field-Enhanced Fluorescence Spectroscopy Biosensing

Gold (Au) is so far the most commonly used plasmonic metal in nanohole-array-based surface plasmon resonance biosensors, due to its excellent plasmonic properties in visible light range and chemical inertness in solutions. However, Au intrinsically absorbs blue-green light heavily, leading to poor plasmonic excitation efficiency below 600 nm. This work explores the possibilities of using various kinds of plasmonic metals (Au, Ag, Cu, Al) or their composites (Ag/Au, Cu/Au, Al/Au) in nanohole-array surface plasmon field-enhanced fluorescence spectroscopy (SPFS) biosensors, aiming to excite different fluorescent dyes (working in different wavelength ranges) with satisfactory plasmonic performance. It is found that SiO2 anti-tarnish-coated Ag nanohole array is suitable and practical for exciting 400–500-nm fluorescent dyes. The Au-coated Al nanohole array supports a very good plasmon resonance for fluorescence excitation in 600–700 nm, whose field enhancement even exceeds that of pure Au. The combination of Au and Ag is found to be superior to pure Au or Ag for the 500–600- or 700–800-nm fluorescent dyes. These findings provide the guideline on selecting suitable plasmonic metals for nanohole-array SPFS biosensors.

Study of coupling efficiency of molecules to surface plasmon polaritons in surface-enhanced Raman scattering (SERS)

Studying the interaction between molecules and surface plasmon polaritons (SPPs) is of great important in understanding surface-enhanced Raman scattering (SERS). While it is known that SERS consists of excitation and emission enhancements, each of them is manifested by several sub-steps which individually also deserve attention. For example, for emission enhancement, the energy from the excited molecules is first coupled to SPPs, which then radiatively scatter to far-field. To understand these two sequential processes completely, differentiating them one by one is necessary. Here, we decouple them and determine the coupling efficiency of molecules to SPPs by using a phenomenological rate equation model. We find the coupling efficiency, defined as the ratio of the coupling rate from molecules to SPPs to the direct Raman decay rate, can be expressed as the SERS intensity ratio and the SPP absorption and radiative decay rates, which all can be determined by polarization- and angle-dependent Raman and reflectivity spectroscopy. As a demonstration, the coupling efficiencies of 6-mercaptopurine to SPPs propagating in Γ-X direction on Ag nanohole array are measured for several Raman emission wavelengths.

Extraordinary optical transmission of multimode quantum correlations via localized surface plasmons.

We demonstrate the transduction of macroscopic quantum correlations by Ag localized surface plasmons (LSPs). Quantum noise reduction, or squeezed light, generated through four-wave mixing in Rb vapor, is coupled to a Ag nanohole array designed to exhibit LSP-mediated extraordinary-optical transmission spectrally coincident with the squeezed light source at 795nm. This first demonstration of the coupling of quantum light into LSPs conserves spatially dependent quantum information, allowing for parallel quantum protocols in on-chip subwavelength quantum information processing.

Panchromatic plasmonic color patterns: from embedded Ag nanohole arrays to elevated Ag nanohole arrays

Plasmonic structural color is one of the most fascinating applications of recently fast developing plasmonics, which is a promising candidate technology for information processing, color displays and optical measurement devices. However, the implementation of plasmonic structural color in modern optical and spectral imaging systems demands strongly a simple fabrication process, low power consumption, and complex color pattern integration. Thus far, tuning of the plasmonic color has been generally achieved by morphology alteration or adjusting the lattice constants of plasmonic nanostructures. Nevertheless, this strategy suffers greatly from high cost and low throughput when designing complex color patterns. Herein, by precisely controlling the refractive indices on two sides of Ag nanohole arrays (NAs) that are embedded between a silica coating and a glass substrate, we are capable of filtering white light into individual colors across the entire visible band. Moreover, the straightforward strategy we propose is compatible with the traditional photolithography process, with which complex color patterns can be easily achieved. We further experimentally demonstrate that these engineered colored samples can be used as chromatically switchable anti-counterfeit tags. We anticipate that the method we demonstrate can provide a new approach for the fabrication of compact color filtering devices.